The present invention relates to the field of combustion chambers for turbomachines, and in particular to the annular walls of turbomachine combustion chambers, which walls have a cold side and a hot side.
The term “turbomachine” is used in the present context to mean a machine that converts heat energy of a working fluid into mechanical energy by said working fluid expanding in a turbine. In the description below, the terms “upstream” and “downstream” are defined relative to the normal flow direction of the working fluid through the turbomachine.
In particular, the present invention relates to so-called “internal combustion” turbomachines in which the working fluid of the turbine includes at least some of the products of the combustion that has delivered this heat energy to the working fluid. Such turbomachines include in particular gas turbines, turbojets, turboprops, and turboshaft engines. Typically, such internal combustion turbomachines include, upstream from the turbine, a combustion chamber in which a fuel is mixed with the working fluid, typically air, and is burnt. Thus, the chemical energy contained in the fuel is converted into heat energy in the combustion chamber, thereby heating the working fluid, and it is the heat energy of the working fluid that is subsequently converted into mechanical energy in the turbine. Typically, such a turbomachine also includes, upstream from the combustion chamber, a compressor that is driven by a rotary shaft that is common to at least one turbine stage in order to compress the air before combustion.
In such a turbomachine, the combustion chamber typically has at least one annular wall with holes for enabling the air that flows on the cold side of the wall to penetrate to the hot side of the wall. Such a wall extends in the flow direction of the working fluid between an end wall of the combustion chamber, where fuel injectors are normally situated, and a combustion gas outlet. The combustion chamber is typically situated inside a gas generator casing, which casing is in communication with the compressor in order to receive the air that has been compressed therein.
In such a combustion chamber, the flow of air through the holes performs several functions. In a first zone referred to as a “primary” zone, close to the end wall of the chamber and thus to the injectors, the wall includes at least one “primary” hole that serves mainly for feeding air for the combustion reaction with the fuel that is injected by the injectors. Nevertheless, the air entering into the combustion chamber through holes situated in a second zone that is further downstream, referred to as the “dilution” zone, serves mainly to dilute the combustion gas, so as to reduce its temperature at the outlet from the combustion chamber, thereby limiting the thermal stresses on the turbine downstream from the combustion chamber.
Nevertheless, in order to increase the efficiency of the thermodynamic cycle of the turbomachine, the trend is to increase the temperature in the combustion chamber more and more. This leads to considerable thermal stresses also occurring on the walls of the shell of the combustion chamber. In order to cool these walls, they may present a large number of cooling holes of small diameter, normally no greater than 1 millimeter (mm). Air entering into the combustion chamber through these cooling holes forms a relatively cold film on the hot side of each wall, thereby protecting the material of the walls from the combustion heat.
In the dilution zone of prior art combustion chambers, there are nevertheless to be found both dilution holes of large diameter, normally greater than 1 mm, for diluting the combustion gas, and also cooling holes of small diameter, no greater than 1 mm, for cooling the walls of the combustion chamber, since the person skilled in the art is of the opinion that it is necessary to have jets of air that can be produced only by holes of large diameter in order to enable them to penetrate deeply into the flow in the combustion chamber so as to obtain better mixing downstream between the dilution air and the combustion gas. Nevertheless, that gives rise to other drawbacks. In particular, those dilution air jets can give rise to large amounts of non-uniformity in the temperature inside the combustion chamber. Unfortunately, for environmental reasons and for combustion efficiency reasons, it is important to ensure that temperature is distributed as uniformly as possible within the combustion chamber. Any temperature peaks therein can give rise to nitrous oxides being formed, whereas fuel can remain unburnt in zones of lower temperature.